1. Field of the Invention
The present invention generally relates to a motorcycle exhaust system and, more particularly, to the motorcycle exhaust system designed to vary the sectional area of an exhaust passage in dependence on the operating condition of the motorcycle combustion engine.
2. Description of the Prior Art
It has hitherto been well known that the conventional motorcycles make use of an exhaust system designed to properly change the sectional area of an exhaust passage in dependence on the operating condition of the motorcycle combustion engine. Specifically, the Japanese Laid-open Patent Publication No. 4-292534, for example, discloses the use of an exhaust control valve capable of continuously changing the opening of the exhaust passage, which valve is disposed at a location upstream of the silencer or muffler.
In the conventional exhaust system disclosed in the above mentioned patent publication, however, the silencer is positioned at a location laterally of the motorcycle rear wheel and relatively close to the motorcycle combustion engine, the distance from the exhaust port of the combustion engine to the exhaust control valve is so small that the exhaust control valve tends to be adversely affected by an elevated temperature of the exhaust gases as the exhaust gases flow through the exhaust control valve. Also, since the exhaust control valve is positioned close to the combustion engine as described above, the exhaust control valve is also susceptible to vibrations induced by the combustion engine.
In addition, since a considerable thermal expansion occurs as a result of the elevated temperature of the exhaust gases, a limitation is encountered to fill up the valve clearance of the exhaust control valve at a low temperature, resulting in difficulty in securing the required valve clearance. Moreover, considering that the output performance of the motorcycle combustion engine is controlled relying on the valve opening of the exhaust control valve, it is not easy to accomplish an appropriate engine control. Furthermore, demands have been made to improve the appearance of the exhaust system since the exhaust control valve and concomitant accessories including, for example, an actuator are exposed bare to the outside.